Google's Android operating system powers a LOT of mobile technology. Along with Apple's iOS, they provide the operating systems for the vast majority of the phones and tablets around today. Apple's iOS is used only on Apple's own devices (manufactured by them).
While Google has it's own "Nexus" brand of devices, which use "pure" or "stock" Android, Google also licences Android for use on other manufacturer's devices, and that's where some potential problems arise, at least with those devices receiving timely Android updates.
The other manufacturers tend to provide customized or "skinned" versions of Android, adding features or appearance changes to the base system. This "added value" approach complicates and delays the update process in many cases, and is one of the complaints about Android security in general. This diversity of Android configurations from the standard version, is often referred to as "fragmentation".
I actually have a Android phone from pay-as-you-go provider Boost mobile, and was a little surprised (but pleased) to get a major update recently (from Android 4.4 to 5.1).
MakeTechEasier has a nice article on this, which goes into more detail.